Feedback on upcoming content and systems is very important to us, and we would like to avoid providing access to those before they are ready, as to not give incorrect impressions as to what we are aiming for and possibly change what that feedback would be if first presented with them in what we consider an acceptable state. Regarding mods which provide access to yet to be released content this is something we strongly discourage. You are free to do it on your own, but by modding your game you may be forfeiting support from Monomi Park if you experience technical issues.
There is no official means to mod Slime Rancher 2 at this time and we do not officially support it.