We did a lotus notes to sharepoint 2010 list migration, one coloumn which has names in it..were suppose to be migrated to sharepoint list people or group coloumn..but since the data in lotus notes is 10 years back some of the people have left the company and are no longer in the active directory...so then it was decided to have those values in a text coloumn..and later on convert that text coloumn to drop down..and this column needs to be selectable.... But again in the coloumn all the names should be there even the names of the people who left the company...how can we have a column which shows all the names including the names of the people who left the compnay but in the drop downbox it shud not show the names of the people who left the compnay... we have a another list(names1) which contains the recent names..
Simplest solution is to put all users (active and inactive) in single 'user list' and use lookup column in migrated list to make reference. However if you add new user to AD you will need to update this 'user list' manually.
Another (and more complex) option is to enable Claims Authentication in mixed mode (Windows and FBA). Only purpose of FBA authentication is to provide store for your inactive users (eg. SQL database). By doing so you can then use OOTB people column because it will list all active users (from Windows authentication) and all inactive users (from FBA authentication).
Here is some links if you like to explore second option: