As noted by @ykaore in posts 3 and 9 of the other thread that question came up, you’ll have to work with the Aitken and accumulation mode species provided by your ISAM run.
Simply put, sum up all the PM species provided in your ISAM output file and multiply the Aitken mode species with PM25AT and the accumulation mode species with PM25AC. Post 9 contains an example how this was done in @ykaore’s case. Note that this will be different from the standard PM25 calculation using the concentration output files because coarse mode species aren’t included in ISAM. @sergey may have additional thoughts.