Enumlist writing new entries to spreadsheet

I have just started using AppSheet and have created an app which works
fine, however I have one question which I am struggling with which may
be very simple but I can’t find an example online.

Mine is a food date checking app and has tables for “Products”,
“Products on display” along with “food category” and also “allergens”.
Each product can have only one category so a simple drop down is fine,
but each product can have multiple allergens. I know I can use enumlist
for the allergens but would like a drop down with multiple selections
that also enables you to add an allergen that is not on the reference
list to the product and at the same time add it to the reference list.
I don’t want to use a virtual column since I would like the people in
the shop to be able to see the current list of allergens on the Google
sheet. Hope this makes sense.