Can I attend Fachkonferenzen 2019 with a re:publica 19 ticket?

No, Fachkonferenzen 2019 are a stand-alone special events, for which a special ticket is necessary. Tickets can be purchased here.

Is it possible to attend re:publica with a Fachkonferenzen ticket?

No, however you can upgrade your Fachkonferenzen-Ticket with a re:publica 19 ticket during the ordering process.

How expensive is the re:publica 19 upgrade for a Fachkonferenz ticket?

The upgrade for attending re:publica 19 (Standard Ticket, 3 days) costs € 100, - (€ 84,03 net) and may be added during the ordering process.