It seems anyone can take part in the Vasaloppet cross country ski race. However, you need to register in advance and pay the feeds as required.

There are different race events, 58k, 42k, 35K, 13k, freestyle, classic or replay. The fees vary from 45 USD to 225 USD depending on the event. You can select the event on the register page. You can read race related FAQ to get more information.