What FAR justification and/or FAR regulations would be most appropriate to include in a J&A to extend the current contract?
Good question. Obviously you will get feedback on your decision as you process your J&A for approval. Based on the facts you provided, I will assume that the supplies and/or services being provided under these contracts is anticipated to continue throughout the national emergency.
If yes, then there appears to be only justification for your scenario and that is FAR 6.302-2 Unusual and Compelling Urgency.
This is clearly an unusual situation. See FAR 6.302-2(b) Application. This authority applies in those situations where- (1) An unusual and compelling urgency precludes full and open competition; and (2) Delay in award of a contract would result in serious injury, financial or other, to the Government. Note: bold font added for emphasis.
Whereas if you read the application at FAR 6.302-1(b), it does not appear that any of the examples or scenarios provided imitate your situation.