"Realtor" is a confusing term in your question, since it only means that someone is a NAR member. Do you really mean a broker or an agent? It's significant because if you're talking about an independent broker, sure - a self-employed, independent broker can offer whatever services they want and are licensed for (and, hopefully, are qualified to perform). If you're talking about an agent who works for a broker, then it's up to the agent's broker to authorize the agent to offer whatever services the broker allows them to perform per their agent contract.
99.9999% of brokers use contracts that don't allow their agents to work on the side performing tasks that require a license but that the broker doesn't sell. Leasing is a common example. I've known many agents who have wanted to become property managers or leasing agents to increase their income, only to be told by their broker that their contract forbids them from doing so.
In other words, if you're currently working with a broker, check with them to make sure it's OK.