Custom training data for BERT

[AlanQuille]

Hi all,
I was wondering if it is possible to use other data than Ontonotes for the training, specifically custom data that one can prepare themselves. If so, how does one do this with BERT? Will BERT convert a txt file to the right format or does one have to convert it to a ConLL file first? Does it have to be ConLL-2003 or can it be ConLL-U? Thank you very much.