You have two options.
1. Using only email forwarder
You can add a mail forwarder for that one email address to google apps mail address on test domain alias.
If you add your_domain.com to google apps, Google will provide you with your_domain.com.test-google-a.com as a test domain alias
Lets say you want email1@your_domain.com to be on google apps, don't change the mx records for the whole domain, just add a forwarder for email1@your_domain.com to forward all mail to email1@your_domain.com.test-google-a.com.
This will allow you to read all mail sent to email1@your_domain.com. inside the Google Apps account, either through Gmail interface, or inside your mail client.
You can then add a email1@your_domain.com. as an alternate email address in Gmail interface, to also be able to send as that email address.
2. Using Inbound mail gateway option in Google Apps
To configure an inbound mail gateway:
Update your domain's MX records so that the highest priority record refers to the inbound mail gateway server.
Configure the inbound mail gateway server to deliver mail to the Google Apps mail servers, for the accounts you want to have on Google Apps, by adding email forwarder to test domain alias.
Sign in to the Google Admin console.
From the dashboard, go to Apps > Google Apps > Gmail > Advanced settings.
In the Organizations section, highlight your domain (top-level org).
In the Inbound gateway box, enter the IP address of the inbound mail gateway server.
If you have more than one gateway server, enter an IP range in CIDR notation or separate each IP address with a comma.
Select the check box Only let my users receive email from the email gateways listed above.
This setting ensures that all incoming email comes through the inbound gateway server and is therefore properly filtered or archived. The Google Apps mail servers will reject incoming mail from any other mail server.
Click Save changes at the bottom of the Email settings page.
Verify that incoming mail is properly delivered.
Once the Time to Live (TTL) has expired for the MX records that you changed in step 1, send an email message to a user in your domain (see Avoid bounced messages after changing MX records for more details about how TTL works). Confirm that (a) the inbound gateway server processes it and (b) the user receives the message in his or her inbox.