Ok... Apple is probably gonna "pass a brick" for this one... and they may change something to break this later (i.e., I don't think you're supposed to be able to open FCPX placeholders in a regular Motion project, which is too bad because I, for one, would love to be to add them to Generator projects!)
It takes some setting up -- but you can use transitions in Motion 5.
FCPX Transitions:
In FCPX, right click on a transition and Open a copy in Motion (automatically creates a [transition name] Copy in the same named folder in ~/Movies/Motion Templates/Transitions/etc...
Go into that folder and drag the copy to the desktop. (You just need the .motr file.)
[For transitions that already exist in your users folder or those you download (make sure you're using a copy/duplicate)]
Change the file type from .motr to .motn
Double click to open in Motion (5).
You will need to copy the Transition A and Transition B placeholders as new Drop Zones. Being careful to duplicate the In and Out points for each transition placeholder for the Drop Zones (also make sure they have the same stacking order as the originals.) The easiest way to do this is to select the Transition, move the playhead to the start of that transition and type Command-Shift-D to create a new drop zone - it should stack right over the selected placeholder.
IF there are effects and/or behaviors attached to those transitions, shift select them and drag them to the Drop Zone copies you created - respectively.]
If there are several groups, select them all and command-shift-G to place them all in a single group.
Give that group the name of the transition (so you'll remember it later).
In the Library panel, select Favorites and at the bottom click the + to add a Transitions folder (double click on the Favorites > Transitions folder to open it [In the "Theme" section])
Drag your single group into the space where content usually appears > your Transition will be saved to Favorites.
Open a new project. You can drag that Tranistion out of your Favorites into your project (or click the Apply button at the top of the Library pane). Upon inspecting the contents that "came over" - you should notice that the Original Placeholders are NOT there!! [You can't delete them from the original transition project anyway, and there is no way to copy them over to another kind of project, that's why you have to duplicate them with new Drop Zones.]
You shoud be able to line up the transition in your project by the Group you created.
You will need to adjust the Drop zones for your content (or trim copies of the video to fit the In/Outs of the transition).
Have fun! 😉