Walkons have become problematic in the title-9 era as they count against your totals and m/f participation ratios.
Most schools have some "recruited walkons" who were kids that interested them from HS but didn't warrant a 'ship for whatever reason? If they improve they might get some $$$ down the line. Eg: maybe a kid who runs a 1:54 in HS and the school thinks they can get him down to 1:49 or so with some development, training et al.
As for kids just calling and saying they want to be on the team even if they won't be in any meets: that sounds a bit iffy. They may let you if they thought you added some depth but I have my doubts it would be allowed by any more than just a few, if that.