There are a few ways to fuse Sarasvati with Agilao that players can use in the DLC. One of the early requests available with Elizabeth is the Velvet Room Door in the Abyss of Time; these little missions can be completed in exchange for various rewards.
Those based on bringing her Personas with certain skills require experimenting with the fusion system, similar to the base game.
While these fusion requests can get a little difficult, they're more than worth the trial and error, but more so if you already have the ingredients list of Personas to hand.
Not only do these grant Aigis additional Personas with a more diverse loadout, but Elizabeth will grant rare and exclusive items in return, like when beating Precious Hand.
Since Sarasvati is one of the first fusion requests, players can choose more than one option. The easiest way to fuse Sarasvati with Agilao in the is by fusing Orpheus with Lamia.