Hi @eric.bunn,
Yes you can. Here is a Python example:
import Rhino
import scriptcontext as sc
key = "test"
value = "Hello Rhino!"
sc.doc.Strings.SetString(key, value)
rc = sc.doc.Strings.GetValue(key)
print rc
new_value = "Hello Eric!"
sc.doc.Strings.SetString(key, new_value)
rc = sc.doc.Strings.GetValue(key)
print rc
You can also do this with C++, C#, or RhinoScript.
– Dale