The first step is to convert your comma separated list into a list object. Split() returns a list of items.
NOTE: column [list bullets] is actually =split.BulletedList()
The next step is to get that list into a multi-select dropdown. There are a few ways to do this. Since I don’t know exactly what your end use is, I can’t suggest one way over another.
However, you caused me to figure out something that I will now use in one of my own workflows, so thanks!! 
(see below)
