Neither Telegraf nor of course Influx are specific to TTN, what you need to do is come up with another scheme for injecting your non-TTN IoT data at one of those points.
Surely those solutions have many examples for a variety of types of sources.
And you can make your other data feeds similar in their content semantics to your TTN ones, ie, same packet contents and even go so far as to give them nominally similar (if perhaps irrelevant) metadata, though establishing a more sensible union of what both offer would probably be the best.