Peace be upon you. According to Grand Ayatollah Khamenei, there is no objection to performing the recommended prayers with the intention of seeking reward. According to Grand Ayatollah Sistani, it is permissible to perform the recommended prayers after the obligatory ones, but they would be considered as qada (making up for missed prayers). Engaging in supplications after completing the prayer is also acceptable, and you may perform what was mentioned in the question with the intention of seeking reward. May you be successful in all good.