Simply adding it shouldn't, as you can remove it with 30 days notice, but if you take it as part of a new bundle with a discounted price, it will re-contract you.
Best to discuss this with Retentions, being very clear about what your requirement is.
Call 150 from your Virgin Media landline, or 0345 454 1111, and take option 1 initially for Virgin Media, then option 4, followed by 5.
Retentions are a UK group who don't work Sundays.