Not sure I totally understand the situation. You are upgrading Sybase software? If so, current statistics aren't affected, and will be there after the upgrade. No need really to necessarily worry about out-of-date statistics after an ASE upgrade. Maybe you could expand more regarding your concern about statistics post upgrade?
In any case, yes, the "optdiag" tool is the right choice to import "column" level stats (those statistics found mostly in sysstatistics). The table/index level stats (mostly found in systabstats) are kept in ASE memory for the most part and flushed to systabstats periodically. So, importing table/index level stats is possible, but they will just be over-written by ASE very quickly with the "actual" statistics from the tables/indexes. As others have mentioned, this works best when the input statistic's ASE version is the same as the target ASE version.