Tourist visa extensions are usually rejected, and they're surely not going to approve it for the purpose of contemplating marriage.
If you're a US citizen and you want more time to consider marriage, apply for a fiancee visa (K-1). That should get her an interview at the US consulate in her country in about 3-6 months. Assuming they gave her a 6 month stay for her B2, she has 4 months left so if you file for the K-1 now she can stay in the US until close to the interview date.
After approval at the consulate, she'll have 6 months to enter the US with the visa. Then after entering the US with it she'll have 90 days to get married to you. If she doesn't marry she needs to leave by the end of the 90 days.
Overall that gives you about a year or more to think about and plan the marriage.
