In the properties tab you are free to change the decimal separator for you primary units freely but this option as of now is not available in the dimension style manager.

The fix is relatively simple. Create a dimension and format it the way you wish it to look. In my case I wish to add an "X" as the decimal separator so in the properties panel I change the decimal separator from "." to "X." Once this is complete you can add this format to the style by using AIDIMSTYLE. once you Activate the AIDIMSTYLE command select the SAVE option and you will be prompted to select the dimension with the style you like. Then it will prompt you for a name for the new style.

Once this is complete simply select your new style from the available styles.

This method is also a handy way to create styles from existing dimension formats which have not be created in your dimension styles.