I'm pretty sure this would NOT be possible and here is why:
You could indeed take the incoming calls from your PSTN line and send them to your Google Voice account, BUT, since the calls would appear to be coming from the same account associated with the Google Voice number, you would essentially be giving callers the ability to listen to your voicemail or to place outgoing calls from your Google Voice account! Probably NOT what you intend.
Even if you could totally disable Google Voice's voicemail (which you cannot), because the call is being placed from your Google Voice account, it would show the Caller ID number of the Google Voice account. But that's only hypothetically speaking because Google Voice doesn't allow you to turn off their voicemail.
The only way I could envision this working the way you want is if you also had a SIP account with a provider that will actually pass the caller ID you send (many won't). You could then send the PSTN call to that SIP provider with the destination your Google Voice number, and it would send the call out to Google Voice, which would then presumably send it back to you. Of course, that would not be free.
The only other alternative would be to actually port your current number to Google Voice, assuming that's even possible (which if it's a landline, it may not be, but even if it is possible it's a rather convoluted and somewhat costly process — the instructions are elsewhere in this forum — and once you do it then Google Voice basically has your number). Or you could port your landline to a SIP provider and forward it from there to your Google Voice number, if you wanted more control. But any solution involving a SIP provider will have you paying per-minute and/or monthly charges.
There might be another solution that I'm overlooking, but if so it's not coming to me right now.