Generally speaking LCAs are usually used to find out how much GWP the project has remaining in order to know how much they need to offset. Carbon offsets are normally what’s used when all other options have been exhausted to offset whatever little GWP that’s left. Therefore it seems rather counter intuitive to model carbon offsets in an LCA. I believe EN 15978 also states that carbon offsets should not be included in scope.
Are you sure you’re not talking about renewable energy purchase agreements instead? If so, please refer to this support post: https://support.etoollcd.com/index.php/knowledgebase/greenpower-renewable-energy-purchase-agreements-in-lca/