Assuming you are only using the Model Manager or the Models directory within the Data Dictionary folder to manage your model, you cannot do so using any normal/supported functionality. In order to ensure consistency and correct behaviour, some changes cannot be made to a live model without downtime / reindexation, in short, administratively complex tasks, and since the Model Manager / runtime model deployment is meant for zero downtime model definition/config, it has that restriction built in.
There are multiple ways to enable indexation using administrative means. One approach involves identifying the content file for your model JSON + XML files inside of your content store, and modifying those files in-place. Once done, you would stop Alfresco + SOLR, delete the model files + index from SOLR, and restart. SOLR will reindex all existing content for your model including the newly enabled model property.
