You can develop custom action in Visual Studio. I have article regarding that.

Workflow (SharePoint 2013, 2010, SharePoint Online) custom action using Sandboxed Solution

The example I gave in my article will not meet your requirement though you will get basic steps of creating a Custom Action.

Areas to Change

• declare two parameter in Element.xml. Fist one will be the input string and set the Direction to In and second one the output string (set Direction to Out).
• Now in the Activity method, modify input string according to your liking and set it in the output string.

Element.xml

<Parameters>
    <Parameter Name="__Context" Type="Microsoft.SharePoint.WorkflowActions.WorkflowContext, 
        Microsoft.SharePoint.WorkflowActions" Direction="In" DesignerType="Hide" />
    <Parameter Name="Input" Type="System.String, 
        mscorlib" Direction="In" DesignerType="ParameterNames" />
    <Parameter Name="Output" Type="System.String, 
        mscorlib" Direction="Out" DesignerType="ParameterNames" />
</Parameters>

Activity method

public Hashtable CustomActivity(SPUserCodeWorkflowContext context, string Input, string Output) {

}

Let me know for any issue!!