Can we start copying data from the middle of the original table.

Is there any way, we can start copying our data from the middle of the original table. As if I have 100K rows in my original table, and I have to add a new column with only last 50k rows (doing alter and purging in a single step).

Can I command gh-ost to start coping the rows from id number 50k ?

Note - I am damn sure that no transaction is going to happen on my fist 50K row, that I need to purge.