You sure can! You'd start by creating Workflow User Groups. Make one for Purchase Order Approvers, and one for Item Approvers. For each, define the User(s) who will be doing the approving. You can have multiple people defined as approvers (and set a Sequence No. so you know who has to approve first).
Then, in the Workflow (let's use the Item Approval Workflow in my previous example; be sure to uncheck the Enabled flag for the Workflow before changing it), look at the first Workflow Step (the response reads "(+) Add record restriction."). Click the response, and in the window that pop up, click on the second row (which reads "Create an approval request for the record using approver type Approver and approver limit type Direct Approver.")
Notice the Options that appear for the selected response. Find the field called "Approver Type" and change the value to "Workflow User Group", and in the field below it, select the Workflow User Group code that you've created for Item Approvers.
Click OK. Enable the Workflow again. Now you have a specific set of approver(s) for Item Approval.
Repeat the process with your Purchase Approval Workflow. Sit back, and take another sip of that delicious coffee. :)
