In order to assign multiple planned order to a single purchase requisition number, you need to make used of the external purchase requisition number range.

Define your external purchase requisition number range in OMDN.

Assign the external purchase requisition group number range to used in OMI3.

Once the IMG settings are completed, you can used MD15 to assign the multiple planned order into a single purchase requisition number.

In MD15, select the planned order you want to convert to a purchase requisition.

Next, click the Convert Online button.

SAP will display the planned order in change mode.

Now, look for the Purchase Requisition fields and supply the Purchase requisition number.

For example, assuming that you assign the external number range 2000001.

In the planned order, you type the first number range 2000001 and item number 10 subsequently, copy and paste the number range but input the item number manually.

It would be like   2000001   10 and save
                          2000001   20 and save.....

The disadvantages is that the user have to track and enter the external number range manually.
Therefore, there is a trade off, between 

1.  having a single planned order for a single purchase requisition number or 

2.  having multiple planned order to a single purchase requisition number.

Planned Order Convert
Planned Order Conversion to Process Orders

SAP Production Planning Books
SAP PP Books - Certification, Interview Questions and Configuration

SAP PP Tips
SAP PP Tips and Production Planning/Control

Best regards,
SAP Basis, ABAP Programming and Other IMG Stuff
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