Hello JohanF ,
Your xml example shows the name of a property... I was getting a little confused by your question because it sounded like you want to set the value of a property (not property name)...
As a minor rule of thumb, it is wise not to allow property names to contain special characters as you are witnessing... These names are being utilized by internal workings of SoapUI and you are begging for issues if you must include special characters for a property name. Property values can have special characters without much issue. Before setting a property name maybe you could scrub it for special characters and eliminate them or replace with inert character (underscore or dash or something). Just my thought... I don't think there is a native solution to what you are encountering.
Regards,
Todd
