Your regular TVs are just getting analog cable. You can use the analog modulator to inject a signal to those TVs. You will probably not be able to get it to work with the TV connected to the cable box (i.e., through the cable box).
You will probably need to split your cable into analog and digital "sections."
1) Cable (A) comes into house from outside.
2) Use splitter (1) splits cable (A) into 2 sections: cables ( B ) and ( C ). You will use cable ( B ) for all digital connections and cable ( C ) for all analog connections.
3) Cable ( B ) runs to your digital cable box (the DVR) and/or cable modems (etc.).
4) Cable ( C ) will be used for analog cable. You should run cable ( C ) to into filter (2) to prevent your modulated channel from entering the common cable grid (going out to your neighbors) and/or interfering with your digital signal going to the DVR.
5) From filter (2), you will run cable (D) to some combination of combiners, splitters, and amplifiers. It is here that you will inject your modulated signal (on a cable channel (100-125)).
6) You can run cable (D) into one half of a two-signal combiner (3) (looks like the reverse of splitter (1) - some are interchangeable, some aren't).
7) Cable (E) runs from your modulator into combiner (3).
8) The output from combiner (3) is cable (F). You can feed this cable into your amplifier/splitter (4). Note that some splitters and amplifiers also have the combiner functionality built in. Sometimes, with a splitter you can also get away with using one of the outputs for an input (but you can't do this with the amplifier unless it's bidirectional).
With this configuration, you will not be able to view the modulated signal on the set connected to the digital box unless you run an additional cable from the amplifier to the TV.
You can try feeding the modulated signal into the pre-digital connection, but you will probably have problems.
And, you really don't have much choice about using filter (2). You are not supposed to feed stuff back out to other people (and you probably don't want to anyway).