No guarantees here, but you can at least try something like the following pseudocode
// DefaultCredentials returns an ICredentials reference
// which can then be used to call GetCredentials,
// and *that* returns a NetworkCredential.
NetworkCredential netCredential = CredentialCache.DefaultCredentials.GetCredentials(..);
// Now, with a NetworkCredential that *might* have a SecurePassword, you
// could see if that would work to create a PSCredential. I haven't tested this,
// and honestly I'm a bit dubious of the prospects, but its at least a thought.
// There may be some translation necessary to even make that SecurePassword work,
// assuming it is even available in your context, to something the PSCredential
// can use.
PSCredential psCredential = new PSCredential(netCredential.UserName,
netCredential.SecurePassword);
As noted, this is untested pseudocode, but hopefully it gives you a push in the right direction. Good luck.