Is it possible to generate an invoice for my lnd without having a connection to my lnd?
Yes this is theoretically possible as described in this video https://youtu.be/bihYeAeNi0g The idea is to have a deterministic way of generating preimages and entangling the lightning node with the point of sale device.

This idea originated from stepan snigirev who went down further and refined it to this idea: https://lists.linuxfoundation.org/pipermail/lightning-dev/2019-June/002009.html

His main idea is that the offline node creates an invoice with a routing hint through the online node and the online node catches the onion and gets preimage information from it with some form of a ECDH key exchange. 

I think both ideas are not implemented in lnd yet but the latter is supposed to be included.