Add Data to Existing Pre Pay plan?

Just trying to see if this is an option; that either the website is not showing me or I am too stupid to find

I have a 2D $19 Prepay Carryover combo, that with my bonus gives me 1GB of data for the month (which carries over if not used)6 months ago this was more than enough, but with the increased data of Pokemon Go, my 1GB a month is being used and I am now eating into my carry over data (technically I am using my carry over first)

As a result of this extra data usage, I suspect that I will run out of data in approx 4 months (I have 5GB carried over)

I am wondering if there is anyway to add data onto my existing carryover data pool that lasts 6-12 months so I can have that buffer I desire (all of the extra data I see on the website only lasts for a month)