Yes it is possible. I've attached an example that supplies credentials to the Salesforce SOAP API and retrieves an authorization token. Note you'll need to add the base URL to the text input tool and the username/password/securityToken to the payload to actually get it to work. One important piece here specific to SOAP is the "SOAPAction" header.
FYI this snippet is part of the Salesforce Input tool (version 10.0/10.1) macro backend - you can see the whole thing by navigating to bin\RuntimeData\Macros\Supporting_Macros\SalesforceInput.yxmc within your Alteryx installation.
