Pull list from column in another table

I created a formula multi-select text field. I want to write a formula that looks to a column of employee names in another table and shows that list of choices to the user.

The employee names are maintained in a different app but if the lookup formula can't cross apps, I'll setup a sync from Master Data to my current app.

I just don't know how to write the formula that pulls a list from a column.

------------------------------
Samantha Sand
------------------------------