The highest voltage digital pot I'm aware of is 36v, and the digital power and ground must be between the high and low rails (and you need to supply V+ and V-, which must be within 36v of eachother and no terminal on the digipot can be outside of that range).
Digital pots are also limited to a few milliamps of current - you need about 100x the current that a digipot is capable of handling.
Hence, I conclude that your proposed use of a digital pot is not a viable approach.
I think what you need is a power supply that allows you to adjust the output voltage electronically... I suspect there are options for this available, I just am not familiar with them.
Also, a word of terminology: A "transformer" has AC as input and output. A transformer cannot be powered by nor output DC. The "old school" way of making a DC power supply was with a bridge rectifier, filter caps, and regulator on the output of a transformer - this method fell out of favor as the more modern switch mode power supplies (SMPS) became readily available, as they are smaller, cheaper, and more efficient. But regardless of the topology, a device that takes in AC mains power, and outputs a DC voltage is not a "transformer" - it's probably a "power supply"