VLAN to include existing network

Hi,
Hopefully someone can help with a question about upgrading an existing network.  I have inherited a couple of separate networks and one of them will be adding WAPs soon and will want traffic split on those for business traffic and guest traffic.  So the thought was to segment the network into VLANs as there are no such thing there now. But there is an older network that is completely self contained now, with no managed switches, that would need to be able to communicate with the WAPs as well.  I do not have control of this network, so switch purchases and changes are not easily done.  So my question, if I setup a VLAN specifically for this other network on the newly configured managed switches, is there a way to connect to it and have 2-way communication with it?  Hoping for some way to tag all traffic between that old unmanaged network and the new segmented one.  And have a cable that connects the 2 so that I can treat it as another subnet.  Thanks for any advice.