Hi Priya,
I have recently encountered the same requirement from my client that would require to perform Key date valuation (TPM1) without reset. In my case, it was a foreign currency valuation for MM and OTC derivatives.
It should be derived business transaction that should be able to post unrealised gain/loss to realised gain/loss. First if you look at TPM13, you can find which update type (in my case, it was DBT_E005 and E006 that retain the sum-up valuation without reset) and could mark it as posting relevant and assign posting specification to post to G/L. DBT will also post the FX gain or loss as a difference from the last TPM1 valuation.
Hope that this helps
Cheers
Taro