Possible to add a single build with changed value onto a preexisting build?
I have always played that this is allowed. However, there are rule variations that might not allow it. You'll have to agree on what rules you want to play with and use those going forward.

As an example, take a look at the rules found at pagat.com - I think this is where you pulled your rule excerpt from. Rule 5 (Adding to a Build) says there are two ways to add to a build. The second one is this:


  You may add to any build, single or multiple, by playing a card from your hand which, either alone or combined with other cards on the table which are not yet in builds, matches the existing capture number of the build, provided that you hold a card which can capture the combined build.


Here it says you can add to a build by utilizing a card in your hand and cards on the table that are not yet in builds. Under this rule, your dad's move would not be allowed.

However, further down, under "Variations", the same page lists this variation:


  Some players treat a single build exactly like a single card of the same value, for the purpose of capturing it or extending the build. For example, if there is a build of 6 (5+A) on the table and a loose 4, a player who holds a 10 can capture the 6-build and the 4, and a player who holds two 10's can combine one 10 from hand with the 6-build and the 4 to make a multiple 10-build.


This would allow your dad to make his move, since the 3 build that you've made would act just like a single-card 3. He could then add his 4 to it to make 7 and then a double 7 with his previous build.

There are many sites out there with Casino rules on them, many with similar small (but important!) variations. Casino does not have as set rules as something like, for example, Bridge. Since it's a family game, you should take a look at the rule variations that exist and decide on what variations you wish to use.