My game doesnt allow the player to open the RPG Maker menu, so... is there a plugin command to call the Skill Tree?

Yes, there are many plugin commands. See the description of the plugin file for more infos. Here is the part about plugin commands.

* ============================================================================

* Plugin Commands

* ============================================================================

* You can directly open the skill tree scene.

*  - TECHTREE [actorId]

* You can prepare the scene using the following commands:

*  - TECHTREE DISPLAYONLY

*      opens the tree in display only mode, similar to the new plugin parameter

*  - TECHTREE PRESELECT [tree uid]

*      preselects a specific tree for the actor. If the tree isnt found, the default selection is used.

*  - TECHTREE SINGLE

*      prevents the user from changing trees

*

* You can reset all techtrees for an actor. 

*  - RESETTECHTREES [actorId] [refund]

* You can reset a specific techtree for an actor by uid. Put in true or false for [refund].

*  - RESETTECHTREE [actorId] [tree uid] [refund]

* You can save the amount of active nodes in a tree by uid in a variable. Put in true or false for [refund].

*  - ACTIVENODES [actorId] [tree uid] [variable Id]

* You can add a techtree during the game:

*  - ADDTECHTREE [actorId] [tree uid]

* You can activate a specific node during the game for an actor using:

*  - ACTIVATENODE [actorId] [tree uid] [node uid]

* You can deactivate a specific node during the game for an actor using (the usually unlearn settings will be applied):

*  - DEACTIVATENODE [actorId] [tree uid] [node uid]

There are also examples in the demo.