For mails, as @planet_jeroen pointed out, imapsync or similar tools are the way to go.
I saw you can import Outlook PST into Webtop, which is only useful if you already use some Windows:
So you may use M$ Outlook with caldavsynchronizer to get anything from sogo to a PST and then you could import mails, contacts and calendars to webtop with webtop scripts on a per user base.
Maybe we could improve the script to take a list of PSTs and mapped usernames.
SoGo can export vcard and vcal but you can’t directly import to Webtop I think.
It would really be a nice feature for Webtop to have more import possibilities, making it easier to migrate to Webtop.
I don’t know if it would work with EAS. Use EAS client with old SoGo first to get everything. Then connect to new WebTop. Just an idea, never tried. Again this is on a per user base.
Oh, you may just need mails, then this post may be completely useless for you
I really should concentrate more on reading