Disclaimer: I have searched for this question, using a metasearch engine, before of even thinking to post a thread, and it seems like nobody has asked this question before me.

Is there any way to not have to manually do this for each torrent site, and instead, for example, type it in a certain way to catch all sites, or some sort of option in the settings to enable this, thus spoofing your user agent for any and all sites you add without any future input to add said future sites?

Example:
Normally you'd have to
random-torrent-site.org
header:%0D%0AUser-Agent: Transmission/1.30X
peerid:-TR1330-12 random caracter
xfer_peerid:-TR1330-12 same random caracter

random-second-torrent-site.org
header:%0D%0AUser-Agent: Transmission/1.30X
peerid:-TR1330-12 random caracter
xfer_peerid:-TR1330-12 same random caracter

random-third-torrent-site.org
header:%0D%0AUser-Agent: Transmission/1.30X
peerid:-TR1330-12 random caracter
xfer_peerid:-TR1330-12 same random caracter

ETC.

Look here: https://support.tixati.com/individual%20tracker%20options

Guest, does it change the peer ID per swarm too?
When spoofing the ID (lets forget about behavioral changes) you must spoof it to tracker announcements (any differences between UDP and HTTP trackers?) and to peers connected to this (private) tracker's swarm. So you don't connect to them with unspoofed client name and peerID.

Thank you for the link Guest.

It's possible using regular expression. Instead of random-torrent-site.org, type regex: followed by a dot.  
Example:  
regex:.
header:%0D%0AUser-Agent: Transmission/1.30X
peerid:-TR1330-12 random caracter  
xfer_peerid:-TR1330-12 same random caracter

If you want to use the same user agent and PeerID with multiple trackers but not all, you can add the trackers using regex: and separate them with this special character |  
There is no need to add the full address of each tracker. Example, for http://tracker.something.com/abcdefghijklmnopqrstuvwxyz/announce, adding something.com or something is enough.
Example:
regex:tracker1|tracker2|tracker3|tracker4
header:%0D%0AUser-Agent: Transmission/1.30X
peerid:-TR1330-12 random caracter  
xfer_peerid:-TR1330-12 same random caracter
xfer_peerid: changes the peerid reported to other peers, while peerid: changes the peerid announced to trackers.  

The same method apply to both.

Don't know if this helps, but it did for me.

*
header:%0D%0AUser-Agent: Transmission/1.30X
peerid:-TR1330-12 random character
xfer_peerid:-TR1330-12 same random character