Hello Leon,
Yes, there is a pull request here:
https://github.com/OctoMap/octomap/pull/116
It directly modified the OcTree class to extend the range, so the change
was not compatible to merge. But it could give you an idea.
The question is though, if you really need to insert your data in a
global UTM coordinate system. Maybe it's sufficient to transform from
UTM into a locally restricted region, which then fits into the default
OctoMap range.
Best regards,
Armin