Can I check in objects that others checked out in Team Coding without a 3rd party version control?
You cannot check in what others checked out as they are locked under the user who checked it out. If you are a DBA you can login as the uer in question and check in the object with the user's schema.
In general, the DBA, Toad schema or any schema with the Team Coding Admin roles can check the objects back in. You can manually do so by going to the Schema Browser | change to Toad schema user | Tables | look for the table TC_OBJSTATUS and under the PSL_CHECKED_OUT column, changed from Y to N. That should check the object back into.