As you may know, Sybase rarely supports any operations that require direct updates of system tables, eg, if you update sysstatistics/systabstats and ASE 'breaks' then there won't be much help coming from Sybase tech support other than "rebuild your database from a) a dump or b) scratch". Now that we have that out of the way ...
While technically speaking there's nothing to keep you from bulk loading data into sysstatistics/systabsats,you'll probably find it a bit easier (and supported by Sybase tech support) if you just use the optdiag tool to download your dress-rehearsal stats to files and then use optdiag to upload those stats at go-live time.
optdiag is a binary that is distributed with the ASE software and resides in the same directory with the dataserver and backupserver binaries ($SYBASE/ASE-1#_#/bin). For optdiag command line details take a look at the ASE Utility Guide