is it possible to add a TMS layer to TileMill to compose a map? If yes, how is it possible to do it?
Thank you very much,
Unfortunately, TMS is not in the list of layer types supported by Tilemill.
A workaround is to use tilemill-reference-layer. It's limited to maps hosted at MapBox though. Another option would be to download the tiles you're interested in and convert it to supported formats though that one could get very messy and troublesome.