The easiest way to accomplish this would be to create an Announcement that forwards to the SIP URI of your Response Group Workflow. Then configure an Unassigned Number for each additional number that you want to route to your Response Group.
You have to create an Announcement via Powershell:
New-CSAnnouncement -Parent ApplicationServer:<Front End FQDN> -Name "Forward to RGS" -TargetUri sip:rgsworkflow@sipdomain.com
Then create an Unassigned Number for each additional Number:
New-CsUnassignedNumber -AnnouncementName "Forward to RGS" -AnnouncementService ApplicationServer:<Front End FQDN> -NumberRangeStart "+18005551234" -NumberRangeEnd "+18005551234"
where +18005551234 is the additional number you want to send to the RGS. If you already have an Unassigned Number range configured, make sure you reconfigure your existing Unassigned Number ranges to exclude these additional numbers so that there are no Unassigned Number ranges with overlapping phone numbers.
New-CSAnnouncement -Parent ApplicationServer:<Front End FQDN> -Name "Forward to RGS" -TargetUri sip:rgsworkflow@sipdomain.com
Then create an Unassigned Number for each additional Number:
New-CsUnassignedNumber -AnnouncementName "Forward to RGS" -AnnouncementService ApplicationServer:<Front End FQDN> -NumberRangeStart "+18005551234" -NumberRangeEnd "+18005551234"
where +18005551234 is the additional number you want to send to the RGS. If you already have an Unassigned Number range configured, make sure you reconfigure your existing Unassigned Number ranges to exclude these additional numbers so that there are no Unassigned Number ranges with overlapping phone numbers.