Q: In Missions, can Armor Modifications be installed onto an armor after purchasing?
For example, a Full Body Armor with Chemical Seal built in has Availability 20, so with a 5/4 contact you must pay +150% to get it. Meanwhile, the various modifications that cost Capacity have their own individual Availability, rather than modifying the base Availability of the Armor. These availabilities do not exceed 12, so would not require extra payment if bought individually afterwards through a 5/2 or 4/4 contact. Thus, adding e.g. NC 6, FR 6, TD 3 would cost either 4.5k base, or 4.5k*2.5=11.250 nuyen, depending on whether Armor can be modified after initial purchase.
Aside from whether or not you'd have to pay extra for the modifications on a high-availability item, a player with e.g. an armor jacket might want to modify it without buying a new one.