See the official documentation: How Site Recovery Manager Computes Datastore Groups
Site Recovery Manager combines devices into datastore groups according to set criteria.
■ Two different datastores contain files that belong to the same virtual machine.
■ Datastores that belong to two virtual machines share a raw disk mapping (RDM) device on a SAN array, as in the case of a Microsoft cluster server (MSCS) cluster.
■ Two datastores span extents corresponding to different partitions of the same device.
■ A single datastore spans two extents corresponding to partitions of two different devices. The two extents must be in a single consistency group and the SRA must report consistency group information from the array in the device discovery stage. Otherwise, the creation of protection groups based on this datastore is not possible even though the SRA reports that the extents that make up this datastore are replicated.
■ Multiple datastores belong to a consistency group. A consistency group is a collection of replicated datastores where every state of the target set of datastores existed at a specific time as the state of the source set of datastores. Informally, the datastores are replicated together such that when recovery happens using those datastores, software accessing the targets does not see the data in a state that the software is not prepared to deal with.
So, if you can manually edit the datastore group, the answer is depend, for example, if you're using consistency group in your storage array and you add a new volume to that consistency group, that change will reflect in your datastore group.