EDIT: I missed the part in the OP's title saying "custom report". This post was for the out-of-box reports.

Not easily. Most, if not all, reports make use of formulas that are populated when ran from sage 100 and they control various aspects of the report and how it is displayed. What you can do though is launch Crystal Reports separately instead of using the "Design" button on the report screen. Then locate the crystal report in either MAS90\Reports or MAS_<company code>\Reports\<Form/Report Task>\<Form Code/Report Setting> and open it in crystal reports application you launched. This lets you make and save changes while keeping it open in the background, after making a change, switch to the report task and make sure the keep open after preview option is checked, select the correct form code/report setting and click preview. Then repeat the steps where you make and save changes and switch back to the report task and preview.

Another way to locate the report file is to launch the report task, select the form code/report setting, click the Design button, close crystal after it loads, then launch Crystal Reports separately and use the Recent list to open the last report file opened, which should be the report task that you just clicked the Design button on.

One thing to be aware of is with legacy modules like Work Order, you can not have the crystal report file open and attempt to process the report, if you do, sage can't open it and assumes it is missing and then copies the default out-of-the-box crystal report file over the one you have open. You will get a prompt when this happens and if you do not then save your open crystal report from with Crystal Reports, you will lose what ever you had in it. So make sure you make backup copies and it doesn't hurt to have a temp directory either.

If the report is located on any module's Custom Reports menu, you can simply open the file (.rpt extension) with Crystal Reports.  Click the Refresh button or press F5 to run the report in Crystal.

Good point, I was only thinking about the built in reports.

Figured it out.  Until now all my custom reports have been employee focused using the PII Wrk file.

I spent a frustrating afternoon yesterday writing a sales report and never tried to refresh from within Crystal because it had never worked in the past!