You can send a single-ended output to an XLR diff input by just tying the minus (-) pin to ground ... now the difference between (+) and (-) is just the (+) signal relative to ground. I've never read the XLR spec ... so I'm not really sure about the various configurations ...

In any rate, you might still get some rejection this way if the (-) pin at the input is not also tied to ground ... I've never tried it or tested it in that configuration.