You can edit the proxy from MCGUI, on Administration tab, by right-click the proxy you want to add datastores to, and select "Edit Client" it will open a window where you can individually add datastore (by check box) you want to on that particular proxy.
One possible solution is available in version 7.1. You can add all datastores to all proxies and force the proxies to only use hot-add.
You can use mccli command to add individual datastore to a proxy
mccli client add-datastore
Add a new datastore to be protected by proxy client
[--datacenter=String]: fully qualified datacenter name in vCenter. Required for vCenter. Not required for esx host.
<--datastore-name=String>: datastore name to be protected by proxy client. Multiple --datastore-name options are acceptable. It should be of the same datacenter.
[--domain=String(/)]: proxy client's domain
<--name=String>: proxy client name
Alternatively if you already have a datastore selection on a proxy and want to copy the same selection on another proxy you could use proxycp
java -jar proxycp.jar --clonemapping --srcproxy SrcProxyName --dstproxy DstProxyName
What about in VBA, when ever new DataStores are added in, are they automatically scanned and made visible to VBA @ Proxies?