Something like this maybe?
str$ = "aaaaaaa"
PRINT str$
str$ = "bbbbbbb"
PRINT str$
PRINT "last printed line:"; str$
Alternatively, as explained here, you can retrieve characters from screen memory by using PEEK at segment &HB800 , so something like this
DEF SEG = &HB800
mychar = PEEK(1) 'etc
You'd have to keep track of on which line was last printed to know where exactly you need to PEEK, so that will probably get very complicated very quickly...
For that reason I recommend you to rethink what it is exactly that you are trying to accomplish here because "screen scraping" like this is usually just a bad idea.