Twins have an open spot on the 40 man and Wade is probably going to be out for a while so it would be smart to add another outfielder on the 40 just in case something happens to Cave and/or another starting outfielder.
As respectfully as I can put this:
You need to understand all the future consequences of what you are suggesting. Ash and Brock are trying to point you in the right direction.
The Twins don't have to add Rooker to the 40 man roster until December of 2020. Once he is added to the 40 man roster, he can't be removed from the 40 man roster without being exposed to other teams.
This is significant information and you need to understand why it's significant.
Once a player is selected in the rule 4 draft (The Draft).If he is signed before he is 19 the club has 5 years before he has to be added to the 40 man roster or he will be eligible for the rule 5 draft. If he is signed after he is 19 (Rooker) the club has 4 years before he has to be added to the 40 man roster or he will be eligible for the rule 5 draft.
Based on this rule in the CBA... Rooker has to be added to the 40 man roster in December of 2020 or we risk losing him to another team.
Once Rooker is added to the 40 man roster... He can't be removed from the 40 man roster without being exposed to other teams.
If the Twins add Rooker to the 40 man roster (early) at this time as you suggest.
Adding him early guarantees (unless he is traded) that he will occupy one of the precious 40 man spots in December of 2019. This is significant because we have a fairly long list of players that need to be added to the 40 man roster or we risk losing them to another team next Rule 5. Rooker on the 40 man early will give us one less spot to protect a player therefore increasing the odds we lose a player to the Giants/Orioles or any team who would love to scoop up any of the top 5 or 6 or maybe 7 on the list below if left unprotected.
Rule 5 Eligible Dec. 2019
In a nutshell... Adding Rooker to the 40 man for simple insurance as you suggest will probably cost us a fairly significant player in the off season due to lack of 40 man space. It would be like trading Rooker for Jorge Alcala or Trading Rooker for a future free agent because they want to keep Alcala.
Adding Rooker before the Rule 5 draft (or at the very least the upcoming trade deadline) would require roster management negligence from Thad Lavine so it isn't going to happen unless Rooker demonstrates value beyond whatever is currently on the 40 man roster. We are talking a Tatis, Vlad Jr, Acuna or Soto type of demonstration and even that isn't going to help get him here now because that type of talent they would probably want to wait until next Mid-April for the extra year of control if he is that talented.
And once you grasp that... you have to look at the current 40 man roster and come to the conclusion that we have a lot of talent performing at a high level on the current 40 man roster and therefore Rooker can't move the needle enough in 2019.
On top of all that: The Twins need Roster Spots for the Trade Deadline additions and you can bet that the Twins will be using the names on the list above as their preferred trade bait because of the limited 40 man roster space in December and the decisions that they will have to make as a result. Adding Rooker just blocks up a spot and we don't have enough spots.
For example... giving Rooker takes a 40 man spot right now. The Twins trade Alcala for Bumgarner. Bumgarner now needs a 40 man spot and you gave it to Rooker. So to accomodate Bumgarner... the Twins will have to release a player.
Now adding Rooker has cost us a current player on the 40 man roster sometime this month AND a potential Dec. 2019 rule 5 eligible player in December.
Ash and Brock are trying to help you. I'd listen to them because it will be a waste of your time and energy wishing or hoping that Rooker appears in a Twins uniform anytime soon.