Join several UV map in one

Unless someone else knows of another way, in Blender you will need to join the objects by going to your panel that lists your groups. Make sure you are in object mode. Select one group that will be on the uvmap by right clicking on the group. Then, select the remaining groups that will be on the map the same way. Once that is completed, make sure your cursor is in the viewport and press Ctrol + J (j = join). Your groups will be joined together. To make sure everything is on the uv map properly: keep that group selected, go into edit mode TAB key. Then press A - the group that was selected will be highlighted. Then look at your uv map. Make sure nothing is overlapping. If all you are wanting to do is export the uvmap, then keep it highlighted in orange, press the uvs button then at the top of the pop-up menu it say to export the uv map. Do so, window appears and then same the uvmap file. If you don't want to keep the items as one group, then go back to the viewport, make sure your cursor is in there, and press Ctrl + Z until all of the groups that were joined are no longer joined.
if you need help with the uvmap, let us know.