How to create a user plan with quota over 2GB?

Here is the answer:
Open Provisioning Manager on MPS01, browse to Namespaces -> Hosted Email 2007, open ValidateUserPlanData_ procedure, click on XML tab. Now you must see procedure's source code ;-)).
Comment out very first <execute> node after <procedureData/> node. It must look like this:

<procedure name="ValidateUserPlanData_" description="Validate the contents of a user plan" access="private" type="read" perfEnabled="0" saveDataForRollback="0" auditEnabled="0">
  <procedureData/>
  <!--
  <execute namespace="Error Provider" procedure="SetError">
    <when source="data" sourcePath="features/prohibitSendQuota[. &gt; 2147483647 or . &lt; 1000]"/>
    <executeData>
      <hr>0x80070057</hr><desc>The prohibitSendQuota parameter must be a positive integer between '1000' and '2,147,483,647'.</desc>
    </executeData>
  </execute>
  -->
  <execute namespace="Error Provider" procedure="SetError">

MPS will ignore your prohibitSendQuota value from plan settings, and you don't have to manually update anything.