If you need to follow the language specification, then there are limited possibilities as Walid described.

If it is more important to be able to specify certain interaction, rather than follow UML (and its specific version), then you can apply a tool that gives you full access to the metamodel. These tools are often called as Language Workbenches. In addition to giving access to the metamodel you may also define the constraints (e.g. OMG uses here OCL), as well as notation, like showing specialized InteractionFragment different than others. In other words, these tools allow you to be on drivers seat.