Yes, it is possible.

First thing that needs to be understood is the app in particular is not the best tool for planning.

The likes of bahn.com and local operators websites are going to be far more accurate.

It is also the case that for using indirect routes you will often need to break the trip down and search in smaller sections to get the local connections, a decent map can help with this.

Also in some places, Spain is particularly bad for this, local train networks just do not show up on any external systems, you must use the specific planner/timetables for that operator to get any details.

For the area you are looking at it is SNCF as far as Hendaye and then narrow gauge Euskotren https://www.euskotren.eus/en from there into the San Sebastian area and as far along the coast as Bilbao.

This ET is OUT of pass-use-but local fares-like only to Donostia (formerly San Sebastian) are justt like 2-3€

Also check seat61.com for more info about how to do these trips-also other ons, like if you even plan to do the most cumbersome trip into PT