Use of sp_addextendedproperty to add descriptions on Tables & columns when Owner NOT dbo but PSEUDEOuser USER

How can I add descriptions to EITHER a table OR a column when the table owner is NOT dbo but DOES have db_owner role?

all of the examples given:

exec sp_addextendedproperty N'MS_Description', N'This is a description', N'user', N'dbo', N'table', N'table_name'

I want to assume that I do NOT know the username so it's not just N'other user' (in the 3rd to the end parmameter)
I really want to use the USER pseudo column and assume the one adding the description is the OWNER (this is a safe assumption to make)

Thanks!