easy way to convert normal fields into custom fields?

Well, you can export the form and open the XML file and try editing it that way to change the field types. For example, I did a quick test with a form that had a single line field and a post custom field. This was the output:

<?xml version="1.0" encoding="UTF-8"?>
<forms version="1.6.4.beta1">
	<form labelPlacement="top_label" useCurrentUserAsAuthor="1">
		<title><![CDATA[Post Fields Form]]></title>
		<description><![CDATA[We would love to hear from you! Please fill out this form and we will get in touch with you shortly.]]></description>
		<confirmation type="message">
			<message><![CDATA[Thanks for contacting us! We will get in touch with you shortly.]]></message>
		</confirmation>
		<button type="text">
			<text><![CDATA[Submit]]></text>
		</button>
		<fields>
			<field id="1" type="text" size="medium">
				<label><![CDATA[Single Input]]></label>
			</field>
			<field id="2" type="post_custom_field" size="medium" inputType="text">
				<label><![CDATA[Post Custom Field]]></label>
			</field>
		</fields>
		<descriptionPlacement><![CDATA[below]]></descriptionPlacement>
		<notification>
			<to><![CDATA[{admin_email}]]></to>
			<subject><![CDATA[New submission from {form_title}]]></subject>
			<message><![CDATA[{all_fields}]]></message>
		</notification>
	</form>
</forms>

You can see that in this simple example the "type" is different and the CDATA. IF you go this route, just do backups of the original XML and maybe some experimentation. Hopefully that could shortcut it for you. Once finished editing, you can reimport the XML file.

Actually, you are in the right place, mostly. At the top of the Import/Export page you should see an option for Export Entries | Import Forms | Export Forms

Ok, the way to do this for anyone with a similar problem is:

download XML
find 'type'
replace 'type="post_custom_field" inputType'

make sure you 'match whole word only' and run throuht the XML on a case by case basis (don't replace all)