You can quickly change chart intervals with TradeStation macros which can by typed, associated with hot keys or associated with toolbar buttons. For example, one could have a toolbar like the following: 

You cannot add to the following drop-down, in the latest TradeStation 9.0 (Update 8521) platform:

Intervals Drop-Down

For the ability to customize the Interval drop-down list, shown at the top, you may want to request such a feature here: https://www.tradestation.com/Discussions/Forum.aspx?Forum_ID=215&selCategory=0