Can I tap in to a interior wall outlet for outside
Yes But keep in mind that the outside outlet must be GFCI protected. 

If the inside outlet is not GFCI protected already you can replace that inside outlet with a GFCI outlet and attach the outside outlet to the load terminals of the GFCI.

To prevent overloads you can do a check for what else is on that circuit and add all the Watts together and divide by 110. This will give you a worst case current draw. If that is less than 90% of the rating of the breaker there is no issue of overload.