There's another way to cope with a single symbol without wasting a precious math alphabet resource (there are only 16 of them).
After these one can choose a particular symbol and give it a sensible name, for example
The optional argument gives the type of the symbol, in this case a binary operation symbol; the first mandatory argument is the name of the font family and the second one the number of the symbol's slot in the font.
Unfortunately, mathabx doesn't provide
.fd files, so one has also to declare the family as explained in Alan's answer, but, since now the mathabx are also in Type1 format, one may scale them:
<-6> matha5 <6-7> matha6 <7-8> matha7
<8-9> matha8 <9-10> matha9
<10-12> matha10 <12-> matha12
Any symbol in any font may be used in math and it will scale properly in subscripts and superscripts.