The STR() function will convert a numeric value into a formatted text "string". You will need to decide what kind of formatting you want to have expressed and assign it to the raw value.
This example takes the Object's raw width, "A", and raw height, "B", and formats the measured value into text strings defined as "String_W" and "String_H" formatted into feet and fractional inches with a precision of 1/16":
The GDL Manual section of the Help file can explain the formatting in depth.
You puzzled me. You said you want to insert a dimension value into a string, did you?
Well, we know the type of a dimension value is numeric and all string function can not deal with numeric type value. So, I think you have to convert the value into a string type first, then you can do what you want.