Quick Alchemy?
 

I saw that there's support for daily Alchemist preparations, but is there a built-in mechanism on the HLO sheet to use Quick Alchemy to convert reagents into alchemical items during the adventuring day?

Not like there is for the ones made during daily prep, which we assume are usually pretty consistent loadouts. Since they're on the fly and could be anything, it's a case where you just need to add the item via the Infused Alchemical Items table and mark down the reagent used.