Extjs have a 'convert' property in Ext.data.Field but there is currently no way to use the feature in the core ametys search tool (see : org.ametys.cms.helper.SearchContentHMIHelper.getRecordMapping).

I propose a patch to enable this feature.
It works as follows:
To add a converter in the property-mapping of a search tool factory just replace the just add a 'converter' node for the property of your choice.

<property-mapping>
    <property id="title" path="metadata/title"/>
</property-mapping>

with this declaration

<property-mapping>
    <property id="title" path="metadata/title" type="array">
        <converter>my.custom.Converter</converter>
    </property>
</property-mapping>

Where my.custom.Converter is your JS convert function (see the Extjs doc)