Hi Sonia,
You can add custom measures to Lead Analysis in Revenue Explorer, but based on your post, it looks like you're using Opportunity Analysis and custom measures are not supported there.
However, we can re-map the standard Opportunity Amount field in Marketo to point to a different field instead of the standard Opportunity Amount field in your CRM. To do this, you just need to send an email to support@marketo.com with the new field information.
TJ