As Keith said, FPP Connect does this for you. However, you should also keep in mind that it is done DIFFERENTLY than what Vixen does.
If I remember correctly, in Vixen, the FSEQ files that are generated for each FPP instance are more or less non-sparse FSEQ files and start at channel "1" for each controller. Thus, when you configure the FPP outputs, you pretty much restart each controller at 1.
With xLights/FPP Connect, the controllers are configured to exactly match the channel numbers that xLights uses. The FSEQ files are "sparse" and will only contain the data for that FPP instance, but they retain the xLights channel numbers. If the first string on controller FPP1 starts at channel 1000 in xLights, it needs to be configured that way in FPP. The good news is that if you have all the controller connection things and such setup in xLights, xLights/FPP Connect can also upload that configuration to FPP.