Is it possible to modify hidden service configuration to always include a certain tor relays ip to be its introduction point given the relay has HSDir set.
Introduction Points do not need to be a Hidden Service Directory.
In theory, you could manually pick introduction points but there is no configuration option to do this, since it's not helpful for anonymity, security or availability.
You may wish to read up on the specifics of the Rendezvous Protocol to better understand what goes into creating a circuit to an onion service.
There's no way to do this, because it will damage the consensus of the network and it's algo's, by the very same reason you can not filter out middle nodes in any way in instance-wide config. Even more - let's say that somehow you've managed to do so, then your service - in a matter of time and number of requests made for it - will be relatively easy fingerprinted, traced and observed(=censored/blocked).
The HSDir role - as the other flags so far - makes absolutely no connection with the way HS and it's client are communicating, because it is just a replica of HS directory, an address book of some kind. So even being able to select the points with HSDir flag will make no difference at all as selecting nodes without the flag specified: the path's are selected by consensus, yes, but by another and completely different algorythms, i.e. it's not influentional at all.