Yes 24 to 6 pin off the shelf adapter exists, but not validated or sold by Dell.  It is validated by users and third party vendor.  [URL] or one from moddiy for 3060 which works for 3070/5070/7070. Dell designs oem 360/500w psu for the series 6 optiplex 7080/7090 which support 10/11th Intel cpu -k of TDP up to 125w.  These Dell psu are backward compatible with series 5 optiplex including 5070.  These higher power psu have 2x4 pin cpu +6 pin mobo connectors and one to two 6 pin connector for gpu (360w has one, 500w two).  You connect one of the 2x4 pin cpu to 5070, leaving the other 4 pin connector alone (5070 cpu does not require additional power).  You can not buy these directly from Dell unless you already own 7080 etc., but they are available through third party.  Least $ on eBay now $75 for the 500w. [URL] or $55 for 360w.  [URL] 5070 mt case or all Dell series 5 tower case uses TFX form factor psu. A standard ATX psu does not fit the case physically. Therefore it would have to be outside the case.  Another option is SFX psu which would fit the case, although you need to custom install it or leave it sit unfastened inside case.
I want to add a GPU to a 5070, but the power supply isn't up to the task. Plus it lacks the PCIe power plug. I found adapters for other dell models, but not the 5070. As you know, instead of an 8-pin socket on the motherboard, it has a 4-pin and a 6-pin. Does an adapter exist to use a standard ATX power supply, or does Dell sell a more capable PSU for this model? It's a great machine, shame for it to be crippled by the PSU.
Optiplex 5070 MT - install an ATX power supply?