Well mel has import of kinds, it's just that mel is just one namespace so import here imports it to the entire mel environment. Mel is more like a command shell than anything else.
Mel autoloads stuff if in point is a procedure named in the script hash, if not you need to source the file.
Clarification, if you have a procedure:
global proc foobar(){....
in file called foobar.mel, that is in any of mels script reposittories. Then the first time you say foobar() mel goes ahead and sources the entire foobar mel file to chek if foobar would be defined there. If not mel will issue a error. after that foobar is loaded for entire maya so all new events calling foobar use the foobar in memory. You can force maya to loud calling source foobar. Wich loads foobar anew, please note:
if you have a file named somethingOther.mel and have a function called foobar inside it and do
foobar();
source somethingOther;
foobar();
PLEASE NOTE: Some people miight expect the firt line to run proc form foobar.mel and third form somethingOther.mel, well that will not happen maya runs BOTH foobar functions form somethingOther.mel. Source goes ahhead and rund first.
Second note: If you ever put procedure inside a mel file and expect to be able to call the procedure at all it MUST be made GLOBALLY available. Yes that means someone might source a file that runs their code isntead but yes thats alslo possibel in python.