Can someone tell me if this is possible, i would like to transfer over 1000+ sms's from a backup (8100) to a 8300 handset... sort of like merge both of the text together...

BES or BIS? because with BES the SMS messages can be stored on the server i believe, so if you activate an other with the same account they should be transferred automatically to the new phone.
and hey, SMS messages on the sim stay on the sim, so are also transferred

ITs BIS and the sms are in backup form..so a single backup file,

any ideas...

try this: go to Options > SMS Text and make sure to set Leave Messages on SIM Card > Yes; then move your SIM to your new BB, save all messages in the Saved Messages folder