devtool add should magically support the source path being a URL, and download the file (ideally into DL_DIR with a .done), extracting to a temporary location to examine the contents, so the recipe has SRC_URI and checksums populated.
The functionality for this is already in recipetool, so this should be trivial to implement.
Just sent a series enabling this - I discovered a few other related issues along the way, so I fixed those too and added some tests.
Implementation merged to master: http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=266ea28183516646195226c9ac3297a5f45d3834 As above there were some other commits around it intended to help this use case as well.
Verified on master: eac61f37e36099f74485dab398b57f3812826d17 Functionality testing covered by oe-selftest testcase test_devtool_add_fetch (1160) from devtool.