The pipeline
<p:declare-step xmlns:err="http://www.w3.org/ns/xproc-error" xmlns:p="http://www.w3.org/ns/xproc" version="3.0">
<p:output port="result"/>
<p:add-attribute attribute-name="att" attribute-value="{ map{'key' : 'value'} }">
<p:with-input>
<doc/>
</p:with-input>
</p:add-attribute>
</p:declare-step>
