How can I get usernames to include the plus symbol (+)?

I'm working on a demonstration site that includes WooCommerce.

As I've been testing with my gmail account, I've been using the plus symbol to denote my testing email address. For example, when buying a product, I'm selecting 'register' at the time of purchase, with an email address like so:

my.gmailaddress+test@gmail.com

The theme I'm using appears to use this email as the username, automatically, and without the option for me to edit it as it's created. The problem comes when the plus symbol is stripped (santisied) as it's turned into the username. As a user I have very little idea this is happening behind the scenes, and I'm more than a little frustrated when I can't login with my email address, for no known reason.

My question:

Can the WP username sanitisation be tweaked to allow for the plus symbol to be maintained?

Or, failing that, can the username be manually set at the time that I register in the purchase process? Is this likely to be a function of the Theme or WooCommerce, or WP itself?

Cheers,
Dave