Does buying a new account extend my datafeed?

It depends:

• Your datafeed has expired — buying a new Instant Funded challenge does renew and extend it: the first month of datafeed included with the new account covers you again (the datafeed is per user, so it covers all your active accounts).
• Your datafeed is still active — nothing changes: there is no extension, and the fee stays due at the next renewal.
• You buy a Savius Evaluation — the funded datafeed is not extended: the Timed Evaluation runs on a separate track, with its own datafeed included in the entry fee, and does not touch the funded per-user datafeed.

For the datafeed price and what it covers, see Are there any additional costs to consider?.