There are adhesive bifocal inserts that you can apply to your current mask - I don't know how that would work as I believe the position of those would be critical.
You can get "gage reader" masks that have different magnification lenses inserted like bifocals, but the folks that run the local dive shop have reported that these inserts are prone to popping off -
There are also some bifocals that are actually ground into the mask lens. I would think that would be a bit more pricey.
Then there are full prescription ground masks available with every enhancement available as there are for reg eyeglasses (e.g. anti-glare coatings, scratch layers, etc...)
in short, you have options....