I would appreciate a possibility to feed the resetssed() function with a custom client seed.
Would that be possible to implement?

It might be possible at some sites, but I have no intention of doing this. People tend to re-use client seeds and that puts them at risk, especially at sites that are not nonce based. When it can, DiceBot specifies at random client seed when resetting seeds.

hmm, ok I understand!
Would it than be possible to send my own API command string via the DiceBot for resetting seed at PD for instance?

Not without re-compiling DiceBot yourself and making the changes you need to its source.

While it sounds like a very interesting idea to allow the users to send custom API commands to sites, due to the wide variations of the APIs of the sites that's in DiceBot, it'll be extremely difficult to implement a function like that.

Edit: Also, there're security implications to consider here. Someone might release a script that changes passwords, tips balances away, locks down accounts etc etc. And since I know people don't read scripts and just copy, paste and start any script that promises profit regardless of how shady the site or person they got it from is, I don't think that's something I would implement.