raid on server

You would have to see if the RAID controller was configured for additional drives. If you try to add the drive outside of RAID the BIOS would need to support it.

Try it the second way and see if you can turn it on.

can i add both new hard disk to the existing RAID?

Only if the controller supports it and was configured that way it depends on the controller. Some won't and some will

Since the server is older, and the RAID is older (RAID 5 and may be software RAID - who knows), it may be faster to back up the server, install new drives and make a new (larger) RAID array.

What you have is a RAID 1 (MIRRORED).  You can not add or change this without taking a full image backup, destroy the RAID and recreate a newer RAID 1 with larger HDD.  Since you said you have free drives You can add them to you server (If they are supported), initialize and format.  Since you have two drives and you may want some fault tolerance, you would have to create a second RAID 1 for these new drives.  
Once created you can move data to this new vol and you should be set.  

You did not highlight what type of role this server is serving.  If it is a file share server I would move the entire share to this new VOL.

Create a RAID1 in the RAID controller.  

WIndows will see it as a new disk in disk management

Set the old disk and the new disk as a dynamic disk

right click the volume on the old disk and extend it (it will allow you to span it to the second disk)

This spans your data across the two disks.  

It is more difficult to get the data out in a Disaster recovery situation, but if you have good backups etc, it's not really a worry.  

If the disks are all close to the same size, when you have the time, backup the system, delete all RAID volumes in the RAID controller and create a RAID 10 (2 * RAID1 striped as RAID0)

This will increase the speed, but give you the same capacity and you will need a minimum of 2 failed disks to kill the array.  In some circumstances it will survive 2 disk failures and need a third to send you scuttling off to recover from backup.

To add to prior advice, make/model of server, controller type.

Depending on where the data consuming the space, and what the system does would help identify a course of action suitable to your situation, setup.

Provided the raid controller includes options to expand ...
Often, raid controllers can use larger drives as replacements, I.e. Using 146GB drive replacing I've of the 73GB, allowing the array to rebuild, then replacing the other 73gb with the larger drive
once the raid rebuilds, the volume on the controller now has unallocated space.....

Adding the new one you purchased, and making sure you have a good backup, the controller may support a transition from raid 1 to raid 10
After that, you would need to use diskpart to expand/resize......

Has thought been given to getting a newer system through purchase of a pre-owned.system that includes higher storage,

Depending on the age of the server, it could ....

thanks ,this the explanation

@ISMAIL

How is the comment from PowerEdge a solution.  He highlighted variables that were not asked and there was not follow up.

Every one of our responses were conjecture, at best.