You can export out your custom made tables in the App Dictionary, into Excel format, then cull them accordingly as shown in examples of Ninja Hospital data sample or at this point onwards in the forum
http://red1.org/adempiere/viewtopic.php?f=45&t=1831#p8826After culling them i.e. L# for List, Y# for Yes/No, D# for Date, Q# for Quantity, A# for amount and T# for text, String is automatic without pre-text. Also beginning with 'Is' is auto Y#, 'Date; is auto D#.
The whole idea behind Ninja is to attract Excel users handling their large data user sets, allowing fast import of their migration data and auto table modelling. The only drawback is i kept is so light and maintainable (for me at least) that i do away with too much spell checking. So please double check your spelling particularly if it looks up core tables. One typo mistake Ninja shall create entirely new tables and you have to manually delete them (Ninja Delete Model may help but not guaranteed).
Ninja is also powerful in custom module architecture, as it forces non touching of core, and visibility in agile resuability. It shall replace 2Pack soon. I intend to make it into a visual wizard if work permits.