How can styles be transferred from one *.XIN file to another?

Use the Copy Preference tool. (Tools > Copy Preference)  If the command is not located under Tools, the command is an Application Add-in. 

 

Open the *.XIN file in InRoads that will hold the “new” styles.  Tools > Copy Preference > search for the second *.XIN file > select the styles/preferences that needs to be copied over to the opened *.XIN file > Apply.