I am wondering if it is possible to place a formula in a column that will
keep track of items entered and save them as a drop down list.

I have a spreadsheet tracking project costs from various vendors. I do not
know which vendors will be used for a particular project, so I am not able to
create a drop down list.

I would however, want if vendor XYZ Coporation was typed, the this vendor
would now appear in a list to be selected at a later time.