If using the REST API, you can pass Secure Parameters in a Sourced Fixlet Action.
Here’s and example of running fixlet ID 4057 from the master action site
<?xml version="1.0" encoding="UTF-8"?>
<BES xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="BES.xsd">
<SourcedFixletAction>
<SourceFixlet>
<Sitename>ActionSite</Sitename>
<FixletID>4057</FixletID>
<Action>Action1</Action>
</SourceFixlet>
<Target>
<ComputerID>15875486</ComputerID>
</Target>
<Parameter Name="username">user</Parameter>
<SecureParameter Name="password">pass</SecureParameter>
</SourcedFixletAction>
</BES>