I assume you have a Linux system.

Pidgin stores OTR keys at ~/.purple/otr.*

I assume, you unpacked TorMessenger directly in your home. Than TorMessenger stores keys at ~/tor-messenger/Messenger/TorMessenger/Data/Browser/profile.default/.

# Copy keys
cp ~/.purple/otr.private_key ~/tor-messenger/Messenger/TorMessenger/Data/Browser/profile.default/
cp ~/.purple/otr.fingerprints ~/tor-messenger/Messenger/TorMessenger/Data/Browser/profile.default/

# Change protocol name
sed -i 's/prpl-jabber/jabber/g' ~/tor-messenger/Messenger/TorMessenger/Data/Browser/profile.default/otr.private_key
sed -i 's/prpl-jabber/jabber/g' ~/tor-messenger/Messenger/TorMessenger/Data/Browser/profile.default/otr.fingerprints

Lastly, make sure, otr.private_key has only one entry per account with no resource name (user@jabber.systemli.org/laptop → user@jabber.systemli.org).