Acording to the docs, you define a custom type converter for this. The type converter will convert a string value for any value of any type.
<constructor>
<param name="param1">
<value value="42" />
</param>
<param name="param2">
<value value="aieou" typeConverter="VowelTypeConverter" />
</param>
</constructor>
http://msdn.microsoft.com/en-us/library/ff660914(v=pandp.20).aspx#config_value