Basically to use the first games saves with the second game both eboot's would have to be set with the same game ID; this is how the PSP associates game saves with the games themselves. In this case, you would have to find the game ID for Suikoden 1 and use that same ID when converting Suikoden 2, and then hope Suikoden 2 works with the different ID.

Of course, this is assuming games from the PSN are set up the same as ones you convert yourself.