I will attend SLC soon. Am I allowed to extend to meet SRR prior for promotion to SFC?

Chicken before the egg.
Selection for promotion to SFC incurs no SRR, thus you can not extend for something that has not happened yet.
Nor do I think you will be allowed to extend at all unless business rules have changed.. I would suspect you will be required to reenlist INDEF..But, thats a question for your Career Counselor.

EDIT: actually after looking at your profile, it looks like you joined in 2006, some 11 years ago... did you not reenlist last time INDEF?

Second Edit:3–16. Indefinite Reenlistment Program

Ar601-280 para 3-16b seems to say you can only reenlist INDEF, not extend...but again check with your Career Counselor.

a. The Secretary of the Army may accept Regular Army soldiers for an unspecified or indefinite term of service IAW 10 USC, 505d. All RA enlisted soldiers with over 10 years active federal service are required to reenlist for an indefinite term unless otherwise exempted elsewhere in this regulation.

b. Eligibility: RA soldiers in the rank of SSG-CSM who are eligible for reenlistment IAW Chapter 3, this regulation,to include those with approved waivers, and have at least 10 or more years AFS on the date of discharge will be required to reenlist for an unspecified period of time. Soldiers with a service remaining requirement will only be allowed to reenlist, not extend if they have 10 or more years AFS, except for humanitarian reasons or pending other personnel actions.

No you can't because you haven't incurred an SRR until your name comes out on the list. Otherwise, you'll extend and if you don't promote the month you expected you'll have to extend again. That would be irresponsible of your Career Counselor.

You need to plan ahead to cover the school and possible promotion at any level!