To get around the dimmer controlling an outlet issue, another option would be to not remove the HCM06 dimmers, and instead hard wire the transformers, i.e. don't use the plugs/outlets at all. Assuming that these transformers are indoors, such as in the basement, each transformer could be connected to power (coming from a HCM06) by cutting off the plug and running the transformer cord through flexible conduit to the outlet electrical box. (This assumes that the existing transformer cord is of the appropriate type for this purpose). Use the appropriate conduit fittings for the conduit-to-transformer-case and the conduit-to-outlet-box connections. This is probably the easiest and lowest cost solution, as it doesn't require the purchase of any new X10 devices. Another advantage of retaining the HCM06 dimmers is that the person may have wanted to have the landscape lights at less than full brightness. The HCM06 (in addition to Leviton scene capability which maybe wasn't being used) has the resume dim feature. So each HCM06 could be set to say 70% for example, and then when the HXC4D sends an ON command, the landscape lights will always ramp up to only 70%. If using halogen or incandescent bulbs in the outdoor low voltage lighting fixtures, running the lights at reduced voltage this way greatly extends the bulb life. I suspect that this is why the HCM06 dimmers were initially installed. It seems as if the HCM06s are used only as receivers (not manually switched on/off) and control is achieved via the 4 HXC4Ds located in various rooms.
So if GM is going to add an additional transformer and set of outdoor lights, he should consider whether or not it is desirable to have dimming capability. If so, it would be better to use another HCM06 as explained above, instead of the 6227. Leviton HCM06 dimmers can sometimes be found on eBay, both new and used.