First, you will have to decide where you want to have the option menus. This is because eschema adds the new fields on a per-table basis. You are quite likely to have to add the option menu in more than one place; for example, adding an option menu to the slips table may mean that you want to also add one to the followups table.
The columns etable, element, etype, eflags, and elength should all be the same for each option menu choice. The eflags field should include `O' (that's a capital letter "oh" not a zero). The etype column should probably be `C' for character (meaning string); using numeric option menu items may break something; Postgres and MySQL do things differently with converting strings to numbers.
The elabel column contains the string which will be displayed as option menu choices, with one exception. The esequence table indicates the order of the option menu choices. The entry with esequence=0 will be used as the option menu label itself, not as a choice.
At present, the changes have only been tested with the slips table simply because that is where I needed it first. I'll be making sure it works elsewhere, too. If you have a preference for what gets added first, send me email. If you find someplace where the option menu items are showing up as text entries, that's a place I need to work on; let me know about it.