QIF importer: allow new commodity namespaces?

[Charles Day]

When a user imports a QIF file containing a new security, a dialog is
presented that asks which GnuCash namespace and symbol to use (for example,
Red Hat would be "NYSE" and "RHT").

Currently the user is not allowed to enter a new namespace using this
dialog. Is this intentional, or a bug?  Adding new namespaces was apparently
allowed prior to 2.1.x, according to bug 336192.

Cheers,
Charles