We have process, SF_Account_From_Salesforce_PF, to pull the value from Salesforce.
1) You have to change the query at following location according your requirement. The file name is AccountQuery.xml where you have to change the column name. Query xml data should be as follows:
<?xml version="1.0" encoding="ISO-8859-1"?>
<tns:queryString>SELECT Id FROM Account where UpsellOpportunity__c = '$$text180$$'
2) Execute this flow within the main flow and set the value in the main flow through context variable. Please refer to this link on how to use context variable within a flow\mapping: http://support.adeptia.com/entries/21453353-how-can-i-set-a-context-variable.
3) Now you have to map the same context variable in the main mapping, through which you are sending the values to Salesforce. And then you are all set.