We would like to know whether we can provide cost extension to a FAR 8 GSA Schedule contract. If yes, please provide FAR guidance.

I don’t know what you mean by “cost extension” or why you want one.  However, some of the standard FAR clauses provide for adjustments or equitable adjustments in contract (or order) terms, including price and delivery terms.

If will be helpful if you can answer these two questions—

1.  What is a “cost extension”?

2.  What facts lead you to want to grant a cost extension?

Your answers will allow readers here to point you to the appropriate FAR clause.  

3 hours ago, Zita Kusi said:

We would like to know whether we can provide cost extension to a FAR 8 GSA Schedule contract

Are you a GSA contracting officer?

No please

1.  What is a “cost extension”?

A modification of an existing contract where there is an increase to the total estimated amount of the award.

2.  What facts lead you to want to grant a cost extension?

We have an existing service delivery contract and another mission wants to add money in order to benefit from the same services.

44 minutes ago, Zita Kusi said:

1.  What is a “cost extension”?

A modification of an existing contract where there is an increase to the total estimated amount of the award.

2.  What facts lead you to want to grant a cost extension?

We have an existing service delivery contract and another mission wants to add money in order to benefit from the same services.

If the other mission wants to "add money" it probably doesn't really want to "benefit from the [exact] same services," it probably wants you to procure MORE of the same type of (what appear to be severable) services, which is likely new work.  I don't see a lot FSS contracts, but I suspect FAR 8.405-1(d)(1) might be relevant to your question.

1 hour ago, Zita Kusi said:

 What is a “cost extension”?

A modification of an existing contract where there is an increase to the total estimated amount of the award.

Are you talking about a modification to an order under a GSA schedule contract or a modification to the contract?

20 hours ago, Zita Kusi said:

 

We have an existing service delivery contract and another mission wants to add money in order to benefit from the same services.

Are you talking about a contract awarded by GSA for some agency and a particular purpose and not a FSS Multiple Award Schedule contract?  I’m guessing GSA made a contract award for some purpose with your company and another agency also wants to use it?

yes, I am talking about a contract that my mission awarded out of a GSA schedule and another mission wants to buy in (or however you want to call it. but they want to add money to benefit from the same services)

Jacques pointed you to FAR 8.405-1(d). which points you to 8.405-6.  That is good.  It seems this new work is outside the scope of your present order, so a Limited Sources Justification (LSJ) will be needed to support your proposed action.  Can you meet the requirements for a LSJ?